Several attempts have been made to reduce the contact stress in gear teeth, because a high contact stress can cause pitting and eventually failure of the teeth. Many of these attempts have been based on the Wildhaber or Novikov types of gearing, in addition to the more common involute forms. In Novikov gears, the profiles in the transverse sections are circular arcs, the pinion being convex and the gear being concave. The problem with circular arc profiles is that they are not conjugate. With conjugate profiles, the angular velocity ratio is exactly constant. In each transverse section, there is continuous contact throughout the meshing cycle and the contact point moves along each profile towards the tip of the driving tooth and towards the root of the driven tooth.
Because a variable angular velocity ratio is not acceptable, Novikov gears are made helical. This means that in each transverse section, only one pair of points come into contact. At any instant, there will be one transverse section where contact occurs and this contact point moves axially along the tooth face as the gears rotate. Such gears are generally noisy and have not been widely used. The same comments apply to Wildhaber gears which are shaped as circular arcs in the normal sections.
More recently, other profiles have been proposed in which there are two or more contact points in each transverse section. Gears having such profiles still suffer, though, from the disadvantages just described.